Thin strontium titanate film which is a paraelectric material, and thin barium strontium titanate film which is a ferroelectric material, have a higher dielectric constant compared to thin SiO2 film and thin SiN film used in semiconductor processes. Accordingly, these materials are considered suitable for designing circuits having smaller areas. These thin films of high dielectric constant are characterized in that their capacitance drops according to the direct-current voltage applied.
Variable capacitance devices are devices that utilize such characteristics of thin films in their elements. Among the known types of such variable capacitance devices is one comprising multiple variable capacitance elements connected in series, where each element is formed by conductor layers sandwiching a thin film like those mentioned above, with one end of each variable capacitance element connected to a terminal used for applying control voltage, and the other end connected to a terminal used for connecting to ground. With this variable capacitance device, raising the control voltage causes the capacitance to drop due to the aforementioned action of the thin film.
Such variable capacitance device comprises at least one variable capacitance element, at least one wiring part having inductor and resistance components, and wiring parts through which to apply direct-current voltage thereto, and accordingly the Q value (quality factor) determined by the capacitance and inductance of the device has frequency characteristics. In addition, generally with a circuit having capacitance and inductance, the resonance frequency of the circuit tends to drop as the capacitance increases.
As described above, the frequency characteristics pertaining to the Q value of a variable capacitance device is not necessarily appropriate for the frequency band of the signals passing through the variable capacitance device. Particularly when a frequency band lower than the frequency band of the signals includes a frequency at which the Q value of the variable capacitance device becomes the maximum, then the efficiency of the circuit that uses the variable capacitance device drops significantly.